This invention relates to magnetic devices, and more particularly, to write heads for use in disc drives.
Recording heads for use with magnetic storage media typically include a writer and a reader that respectively record and detect magnetic domains in a disc that spins below the head. The writer in a conventional recording head can include a coil of one or more turns wrapped around a soft ferromagnetic yoke. Writers operate by passing an electric current through the coil, which produces a magnetic field that aligns the yoke magnetization along the field direction. For a longitudinal writer, a magnetic field extends mainly between the pole tips but also partly into the media. For a perpendicular writer, a soft underlayer is typically employed in the storage media such that the write field extends between the pole tip and soft underlayer. When the write field exceeds the coercivity and demagnetization field of the media, a domain forms with its magnetization aligned along the write field direction. These domains form the bits of digital data that are detected with the read head.
There are significant physical challenges in trying to achieve an areal density of 1 Tbit/in2 with magnetic recording. Foremost among these will be the write process, in which magnetic domains, or bits, are created in the media. At these densities, the two primary obstacles facing the write process are the use of materials with large magnetic anisotropy in the media and the lack of soft ferromagnetic materials having Bs greater than 2.4 T. The first obstacle is necessary to ensure thermal stability of the media. The average volume of the grains will have to decrease in order to maintain the same media signal-to-noise ratio, which is roughly set by the number of grains in a bit cell. However, in order to avoid superparamagnetism, the magnetic anisotropy has to commensurately increase so that the grains are ferromagnetic and stable over a time scale of years. The net result is that very large magnetic fields will be needed in order to orient the grains and record bits in the media. Since the write field is intimately related to the saturation moment of the pole tip material in the recording head, very high moment materials will be needed to switch the orientation of the media magnetization. Researchers are already using materials with Bsxcx9c2-2.4 T for 100 Gbit/in2 recording, and the saturation moment requirement for conventional recording at 1 Tbit/in2 is likely to be beyond any known material.
These challenges are well known in the data storage industry and alternatives to conventional magnetic recording, such as thermally assisted writing, have already been proposed. However, the best writer technology for 1 Tbit/in2 is still undetermined. For instance, researchers are also investigating ways to switch the magnetization of thin film media that do not rely on thermally assisted processes. These approaches differ from conventional recording in that the alignment of the magnetic field with respect to the media magnetization is significantly different than 0xc2x0 or 180xc2x0. The ultimate goal is to switch the media magnetization using fields that are less than the anisotropy field, Hk.
There is a need for a magnetic write head that can overcome the limitations of existing write heads to achieve increased areal data densities in magnetic recording media.
This invention provides a write head for writing information bits to magnetic storage media. The write head comprises a first write pole for producing a first magnetic field in a first direction and a second write pole for producing a second magnetic field such that a combined field from the first and second write poles lies either substantially along the first direction or in a second direction that is substantially orthogonal to the first direction. The second write pole includes a free layer having magnetization controlled by spin transfer torque, and a first spacer positioned between the free layer and the first write pole. The write head can further include a pinned layer and a second spacer positioned between the pinned layer and the free layer.
The invention also provides a disc drive comprising, means for rotating a magnetic storage medium, and means for positioning a write head adjacent to a surface of the magnetic storage medium, wherein the write head includes a first write pole for producing a first magnetic field in a first direction and a second write pole for producing a second magnetic field such that a combined field from the first and second write poles lies either substantially along the first direction or in a second direction that is substantially orthogonal to the first direction. The second write pole includes a free layer having magnetization controlled by spin transfer torque, and a first spacer positioned between the free layer and the first write pole. The write head can further include a pinned layer and a second spacer positioned between the pinned layer and the free layer.
The invention further provides a method of writing to magnetic storage media, the method comprising using a first write pole and a second write pole to produce a first magnetic field in a first direction to create a first change in the direction of magnetization of magnetic domains in the magnetic storage media, removing the first magnetic field, and using the first write pole and a second write pole to produce a second magnetic field lying in a second direction substantially orthogonal to the first direction to switch the direction of magnetization of the magnetic domains in the magnetic storage media, wherein one of the first and second write poles includes a free layer having a magnetization controlled by spin transfer.
The first magnetic field can be oriented in a down track direction or a cross track direction. While each of the first and second magnetic fields cannot be arbitrarily small, they can have a magnitude less than the magnetic anisotropy of the magnetic storage media.
The magnetic fields can be produced by applying current pulses to the first and second write poles to produce first and second magnetic field pulses. The first and second field pulses can be separated in time by a period less than the relaxation time of the magnetic storage media. Alternatively, a sequence of current pulses can be applied to either the first or second write poles, wherein the current pulses are timed to suppress spin precession in the corresponding write pole.